[docs] [R-package] improve R-package docs on testing

[jameslamb]

This PR proposes some small improvements to the R package README.

• note that R package tests are run on GitHub Actions (not Travis)
• change instructions for getting package coverage to use source builds of the CRAN package
  • for someone checking test coverage, I think using the more-likely-to-work installation method is a lot more important than using the cmake-based method that produces a more efficient lib_lightgbm
• change test coverage script to only care about tests (not examples or vignettes)

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.